Method for controlling the irradiation of polyethylene



as a result of variations in the irradiation dose.

instances, the dyes incorporated did show some change, but could not be rendered "colorlessby 'additionalirradia- United States Patent 'Ofiice 2,830,943 Patented Apr. 15, 1958 METHOD FOR CONTROLLING THE IRRADIATION F POLYETHYLENE Kenneth J. Mackenzie, Jr., Pittsfield, Mass, assignor to General Electric Company, acorporation .of New York No Drawing. ApplicationNovember 3, 1955 Serial No. '544,837 1 Claim. (Cl.'204--154) This invention is concerned with a method for visually controlling the level of-radiation introduced into polyethylene by means of irradiation of the latter with a source of'high energy radiation, such as high energy electrons. More particularly, the invention is concerned with a proc- "ess for determining the degree ofradiation introduced into polyethylene as it passes through a beam of high energy radiation, which process comprises incorporating in the polyethylene prior to irradiation with the high energy radiation, a compatible dye, specifically Sudan III.

In the copending application of Elliott J. Lawton and Arthur M. Bueche, ,Serial No. 324,552, filed December 6, 1952, there is disclosed and claimed a method for the treatment of polyethylene with high energy radiation,

specifically-high energy electrons with a high voltage accelerau'ng apparatus such as that using a resonant transformer or Van de Graaf accelerator, for the purpose of rendering the polyethylene insoluble in solvents in which it is ordinarily soluble, and infusible at elevated temperatures. The equipment designedfor irradiating the poly- Unexpectedly, I have discovered that when a certain "dye, specifically Sudan III, which has the formula incorporated inwsmall amounts in polyethylene, it imparts color to the system which is sensitive to radiation by the electron beam. Such acombination of dye and polyethylene can be advantageously. used for radiation do simetry so that individual pieces of dyed polyethylene can give quick, visual evidence of the irradiation dose inoduced into the polyethylene. In the form of continuous tapesor films, the color change can be used to .con-

trol the irradiation of the tape or :film, either visually or spectrophotometrically. What is particularly significant is the factthat this particular.dye,-after.use as irradiation control, can be bleached to an essentially colorless state by further irradiation with doses which do not adversely affect the properties of the irradiated polyethylene.

It was entirely unexpected and in ,no way could have been predicted that this particular dye when incorporated in the polyethylene would impart tothe polyethylene the ability to be irradiated in such a way that control of the irradiation dose could be maintained within ,fai-rly narrow limits, because similar dyes such as aminoazobenzene, henzidine, methylene blue, and benzopurpurin failed to undergo any visual or significant change in shade or color, In some tion which would not adversely affect the polyethylene.

2 Some dyes changed color so rapidly and at such low dose that they were of no practical use for control irradiation of the polyethylene with high'energy electrons.

In addition, unexpectedly-it was found that some dyes, such as Indophenol, when" incorporated in the polyethylene, were not satisfactory 'for the purpose because they were not stable. Thus, Sudan III, when incorporated in the polyethylene was quite stable to sunlight and ultraviolet light for long periods of time, and no undue precaution had to "be taken when storing samples of polyethylene containing Sudan III incorporated therein, either before orafter irradiation with high energyelectrons. In contrast to-this, Indophenol, although it did show certain changes in color when irradiated with high energy electrons, was relatively unstable in the presence of sunlight and ultraviolet light so that it became bleached by ultraviolet light giving variations in color in the film samples,

. depending upon thedegree of contact with the ultraviolet bleeds out at concentrations light. Thus, its value as an irradiation control indicator was materially reduced because of this defect.

The reasons for the change in color of the Sudan III in the presence of the high energy electrons are not clearly understood. However, it is believed that unexpectedly the change in color may be due to a reduction of the dye by hydrogen liberated during the irradiation of the polyethylene.

The manner whereby the present invention may be practiced is relatively "simple. It is only necessary to incorporate the dye in the polyethylene employing amounts which maintain the dye in a compatible state and yet are sufiicient to show the changes in irradiation dose. Ingeneral, it is only necessary to mill the dye into the polyethylene on a roller mill used advantageously at a temperature of about to C. and insuring that complete and homogeneousdispersion of the dye in the polyethylene is attained. Thereafter, if desired, the polyethylene can be 'sheeted into the form of sheets or tapesand subjected to-irradiation with a high energy radiation, e; g., high energyelectrons.

The amount of the dye incorporated will vary with such factors .asthe molecularweight ofthe polyethylene, the limits of the'radiationdose to which the polyethylene may be subjected, etc. In general, I may use from about 0.001 to about0.5%, by weight, of the dye, based on the Weight of "the polyethylene. When using the Sudan III in concentrations of 0.1% and higher, the color changesfrom red to orange to pale orange with a dose in the range of 5 to 15 10 R (roentgens'). The color change is proportional to the "dose. At a concentration of 0.05% Sudan III, the color changes to orange in the range of between 0.1 to 5X10 R, and then bleaches in the range of from about 5 to 15 x10 R. At a concentration of about 0.02%, the color is bleached at a dose of around 5 1'0 R. As pointed out above, the color changes are proportional to concentrationof dye and total Exact dye concentration depends to some extent on the color changedesired, that is, whether to obtain a change over a wide spectral range orwhetherit is desired tobleac'h "the color out completely. Concentrations on the order of 0.05% to 0.1%, by weight, are generally preferred. Higher concentrations would require greater doses to obtain noticeable change and the dye usually much above 0.1 to 0.2%

concentration.

The irradiating apparatus used in irradiating the dye as well as in Cole application, Serial No. 437,477, filed' June 17, 1954, and assigned to the same assignee as the present invention. By reference, these two applications are made part of the disclosures of the present application not only for the irradiation apparatus described therein, but also for their description of the polyethylene which can be employed in the practice of the present invention. As pointed out in the aforesaid Cole application, the polyethylene may range in molecular weight from about 2,000 to about 30,000 or more, for instance, as high as 35 to 40,000, especially when made by new processes requiring low pressures and temperatures such as that used to manufacture Marlex polyethylene made by Philips Petroleum Company, and polyethylene manufactured by the Ziegler process described in U. S. Patent 2,699,457, issued January 11, 1955.

In order that those skilled in the art may better understand how the present invention may be practiced, the following example is given by way of illustration and not by way of limitation. All parts are by weight.

In the following example, the polyethylene used was Bakelite DYNH having -a molecular weight of about 23,000. The apparatus used for irradiation purposes Was the same as that described in the above-mentioned Bueche and Lawton application and is also described in Westendorp Patent 2,144,518 as well as in the magazine Electronics, volume 17, pages 128-133 (1944).

Each of the dyes mentioned below was incorporated in the polyethylene in stipulated amounts by milling the same. Thereafter, sheeted samples of the dye-containing polyethylene were obtained by molding about 30 to 40 grams of the dyed polyethylene in a 5" square picture frame mold to give sheets of about 60 to 100 mils thick. Strips of the dyed polyethylene 1" X 5" were cut in half. One strip of each dyed sample was irradiated with a 5x10 R and the other strip was irradiated with 10 10 R.

Example 1 In this example, samples of polyethylene containing 0.2% by weight thereof of Sudan III, as well as the same weight of aminoazobenzene, were subjected to the two above-mentioned radiation doses. The polyethylene containing the Sudan III prior to radiation was red. As a result of irradiation with 5X10 R and 10x10 R, it was found that the Sudan III at 5x10 R was a light orange brown and at 10 10 R was almost colorless.

When the concentration of Sudan III was decreased to 0.1%, the bright red color in the polyethylene was changed to light orange over the range of l to x10 R; steps of 1 10 R could be readily distinguished. At a concentration of 0.05% of the Sudan III, the higher doses caused complete bleaching of the color while at a concentration of about 0.02%, bleaching occurred at about 5 X 10 R.

The sample containing the aminoazobenzene was un-. satisfactory for control purposes in determining irradiation doses because the color faded very gradually rather than changed color over a wide irradiation dose range up to as high as 10x10 R.

In general, tests have shown that with Sudan HI, concentrations of the dye in the range of 0.005 to 0.1%, by weight, appear to be best. The higher concentrations show residual color upon irradiation within this range. As the concentration is lowered, the color can be bleached at doses within the ranges of about 1 to 15 X 10 R. It is thus possible to obtain a dye concentration so at a given selected dose will cause bleaching, and this would give a simple minimum dose control.

It will, of course, be apparent to those skilled in the art that other concentrations within the optimum ranges IDecember 1951.

143,150,294, 236-238; September 1954.

tion, the types of polyethylene which may be used with the dye may also be varied within the spirit of the invention. The presence of polyethylene stabilizers and of finely divided fillers, such as, silica fillers, which are inert in the presence of the dye, is not precluded.

The dye-containing polyethylene can thus be controlled as far as irradiation dose is concerned on a continuous basis and if desired any traces of color remaining in the polyethylene can be removed by'treatment with a high irradiation dose to cause bleaching of the dyein the polyethylene. However, ordinarily this is not necessary because the slight color usually present in the irradiated polyethylene is often of a pleasing nature and may very well lend itself to various uses in the arts. a j

The polyethylene-dye system described aboveinaddition to being used for means for controlling .thedose of polyethylene may also be used as radiation dosimeters. Included among such are static dosimeters such as the personnel type (badges, etc.), area type (to measure the long term radiation in given points in the area), special types for measuring radiation beam patterns, etc.

Polyethylene containing Sudan III can be used in electrical applications without any apparent diminution in properties in the polyethylene, nor is there anyapparent eifect on the heat-aging of the polyethylene due to the presence of the dye. It should be kept in mind that the color changes induced by irradiation are proportional to the concentration of the dye and to total radiation dose, and therefore some evaluation of the effect of the dye on the polyethylene will be necessary before one can decide upon any particular dye concentration to be used in connection with anyspecific radiation dose. Determination of this is a relatively simple matter and once evaluated can be used continuously in the irradiation of polyethylene to control the irradiation dose within fairly narrow limits. Thus, if one employs a concentration of, for instance, Sudan III which gives a certain shade within a certain dose and while the said dose is being applied to the dye-containing polyethylene, if the shade should change, it will be apparent to the operator that adifierent irradiation dose is being introduced into the polyethylene, and this will require adjustment of the high .yoltage accelerating apparatus to bring it back into line with the desired dose. The visual color change is quite apparent and rapid so that there is no substantial loss of polyethylene due to the fact that an improper dose has been introduced into the polyethylene;

Although Roentgen units have been employed in the above description, it will be apparent that an alternative terminology can be used and in place of Roentgen units, one can employ the term Roentgen equivalent physical or more briefly REPJ 7 I I What I claim as new'and desire to secure byfLetters Patent of the United States is: I

The process for continuously controlling and determining the irradiation dose being introduced into polyethylene irradiated with high energy electrons emanating from a high voltage accelerating apparatus, which process comprises incorporating in the polyethylene from 0.001 to 0.5 percent, by weight Sudan III, based on the weight of the polyethylene,,and thereafter subjecting the dye-containing polyethylene to irradiation with high energy electrons within a dose range of from about 1X10 REPs to 15 10 REPs thereby causing a chemical reduction of the Sudan I II dye in the polyethylene to produce a decreased color intensity proportional to an increase in the irradiation dose;

References Cited in the file of this patent' Day: Nature, vol. 168, pages 644, 645; Oct. 13,

Henley et al.: Nucleonics, vol. 9, No. 6, pages 62-66;

r Sun: Modern Plastics," @1132, pp. 141 144, 146, 

